The current trend in medical intervention favours a less invasive approach with a tendency towards localised therapy. Surgical probes for minimally invasive (MI) access can be broadly categorised into two main groups: endoluminal and percutaneous probes. Endoluminal probes (such as endoscopes and endovascular catheters) exploit natural orifices and accessible vessels within the body to direct a tool (e.g. optical fibre, tissue sampler, etc.) to the target. Their use has proved very successful in a few key specialties, such as in gastrointestinal surgery, but the range of application areas for endoluminal access is limited. Conversely, percutaneous (i.e. through the skin) instruments are widely used across the full spectrum of “invasive intervention”, as they are highly versatile with regard to the entry point and access route to a chosen soft tissue target. Current procedures which involve percutaneous insertion of needles and catheters range from blood/fluid sampling, tissue biopsy, catheter insertion, ablation and brachytherapy to deep brain stimulation and diagnostic imaging, with new approaches appearing in the literature at an ever increasing rate.
Current percutaneous instruments can be classified into two main groups: thick and non-flexible probes (e.g. biopsy probe and laparoscopes) and thin and flexible needles (e.g. a brachytherapy needle). Thick and non-flexible probes have the advantage that they can be pointed to the target with the aid of a visualisation system (e.g. ultrasound) and will not deform under load. Their manipulation, however, causes significant pressure on the tissue, which limits the surgeon's degrees of freedom. Conversely, thin and flexible probes tend to be less damaging to the surrounding tissue, but deflect and buckle against tissue resistance, resulting in placement accuracy which is inversely proportional to the depth of the target. Additionally, both groups suffer from the same underlying technological and functional limitation: they cannot be guided along curvilinear trajectories. This is a fundamental drawback of percutaneous probes, which limits their application to surgical procedures where a straight line approach is viable.
Three main research lines have previously been followed to solve the needle steering problem. Recent studies have shown that, as a flexible needle with a bevel tip is pushed through soft tissue, the asymmetry of the tip itself causes the needle to bend along a curved trajectory with a single radius. There has also been demonstrated a control strategy to alter the trajectory of the bevel tip during insertion; proportional control of the curvature of the trajectory can be achieved via duty-cycled spinning of the needle itself, where the approach angle of the needle can be controlled through a cyclical rotation along the longitudinal axis. The main shortcoming of the bevelled-tip approach to needle steering, however, pertains to limitations in the cross-sectional diameter of the needle, which is inherent to the steering mechanism itself. Since trajectory control is achieved through duty cycling, the material which the needle is made of needs to be sufficiently stiff to enable the transmission of a torque along the entire length of the needle (which explains the choice of a nitinol stranded wire), while the diameter should be small enough for the needle to be flexible and thus able to bend (currently 0.28 mm). Given the contrasting requirements of a stiff structure and a highly flexible needle, such limitation on the maximum outer diameter cannot be exceeded.
In an alternative approach to needle steering a standard needle is steered along a curved trajectory inside soft tissue (e.g. turkey breast muscle) by applying a moment at its base. The insertion trajectory is planned by modelling tissue and needle interaction and deformation using a simplified spring-damper approach. However, since the targeting performance relies upon tissue stiffness and resistance, the range of applicable soft materials where such control strategy can be used is limited. The application of this approach on soft and delicate tissues such as brain would result in severe tissue tear.
The third and final approach to needle steering is based on combining pre-curved concentric tubes, which can be rotated and extended with respect to each other to control the tip position and orientation. While this approach poses a relatively simple control problem, as the tube segments and their interaction can be modelled using simple beam theory, the range of available trajectories achievable with any one needle embodiment is highly limited. Since each pre-curved tube has a “fixed” geometry and radius of curvature, a trajectory with controlled variable curvature is impossible by design. In addition, the number of curves in the path is irreversibly tied to the number of segments (e.g. three concentric tubes can only produce a curved path with two constant radius curves), which limits the range of applications for which the probe would be suitable.
In summary, while these research efforts have substantially advanced the state of the art, all solutions proposed to date have limited application scope. Hence, there still exists a clear need for new, versatile technology that will enable percutaneous interventions in the body to be executed with accuracy and with minimum disruption to the surrounding tissues: a thin and flexible working channel which can be accurately positioned anywhere within the body, for application to a variety of surgical applications.